How does Go handle goroutines that make blocking syscalls — what happens to M and P?

When a goroutine makes a blocking OS syscall (file I/O, sleep), Go must not stall its P — other goroutines need to continue running. The runtime handles this through handoff: the P detaches from the blocked M and continues execution on a new M.

// What happens when goroutine G1 calls os.ReadFile (blocking syscall):

// BEFORE SYSCALL:
// P1 → M1 (running G1), G2/G3 in P1's local queue

// ENTERING SYSCALL:
// 1. M1 enters the OS kernel for the syscall
// 2. Runtime detects M1 is in a syscall (can't schedule on M1)
// 3. P1 detaches from M1
// 4. P1 acquires or creates a new M (M2)
// 5. M2+P1 continues running G2, G3 ...
// 6. M1 is a blocked kernel thread — parked in the OS

// AFTER SYSCALL COMPLETES:
// 7. M1 returns from the kernel; G1 is now runnable
// 8. M1 tries to acquire any idle P
// 9a. P available  → M1+P runs G1
// 9b. No P idle    → G1 to global run queue; M1 parks

// WHY NET I/O SCALES BETTER:
// Go wraps all socket I/O in non-blocking syscalls + epoll/kqueue (netpoller)
// A goroutine doing net read → parks immediately (no M blocked in kernel)
// When data arrives → epoll wakes → goroutine added to run queue
// → Net goroutines hold no M while waiting for I/O

// Practical consequence:
// net/http server: 100K concurrent connections with ~GOMAXPROCS threads
// File I/O: each blocking os.ReadFile ties up an M — scale carefully

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